Data which is recorded in an optical disc is reproduced by irradiating the rotating optical disc with a light beam having a relatively weak constant light amount, and detecting reflected light which has been modulated by the optical disc. On a read-only optical disc, information in the form of pits is recorded in a spiral manner, previously during manufacture of the optical disc. On the other hand, in the case of a rewritable optical disc, a method such as vapor deposition is used to deposit a film of a recording material which allows for optical data recording/reproduction, on the surface of a base on which tracks having spiral lands or grooves are formed. In the case where data is to be recorded on a rewritable optical disc, the optical disc is irradiated with a light beam whose light amount is modulated in accordance with the data to be recorded, thus causing local changes in the characteristics of the recording material film, whereby a data write is effected.
Note that, the depth of the pits, the depth of the tracks, and the thickness of the recording material film are small relative to the thickness of the base of the optical disc. Therefore, any portion of the optical disc where data is recorded constitutes a two-dimensional surface, and may be referred to as a “recording surface” or an “information surface”. In the present specification, considering the fact that such a surface has a physical size along the depth direction, the term “information layer” is employed, instead of the term “recording surface (information surface)”. An optical disc includes at least one such information layer. Note that, one information layer may in reality include a plurality of layers, e.g., a phase-change material layer and a reflective layer.
In order to read data from the optical disc and/or write data to the optical disc, an optical disc device including an optical pickup is used. The optical pickup includes a light source for emitting a light beam, an objective lens for converging the light beam emitted from the light source on the optical disc, and a photodetector for measuring, when the optical disc is irradiated with the light beam, an intensity of light reflected from the optical disc.
In recent years, as the optical disc, a Blu-ray Disc (BD), a Digital Versatile Disc (DVD), and a Compact Disc (CD) are widespread. The BD, the DVD, and the CD have equal thickness and diameter in their entireties, but are different in physical structures as well as in the distance from a surface on a light incident side of the optical disc (front surface of the optical disc) to the information layer. In addition, the reproduction and the recording of data with respect to the BD, the DVD, and the CD are performed with light beams having blue, red, and infrared wavelengths, respectively. In order to support all the BD, the DVD, and the CD by one optical disc device, the optical pickup includes at least one light source for selectively emitting the light beams having blue, red, and infrared wavelengths.
As described above, the BD, the DVD, and the CD have different distances from the front surface of the optical disc to the information layer, and hence different spherical aberrations occur with an ordinary objective lens. In this specification, the distance from the front surface of the optical disc to the information layer is referred to as “light transmitting layer thickness”.
FIGS. 7A, 7B, and 7C are diagrams schematically illustrating three kinds of optical discs 200 each including a light transmitting layer 5 having a different thickness, and a light beam converged by an objective lens 100. The optical discs 200 of FIGS. 7A, 7B, and 7C correspond to a BD, a DVD, and a CD, respectively. The thickness of the light transmitting layer 5 is a distance from a surface on a light incident side of the optical disc 200 to an information layer 50. As can be seen from the figures, the light beam that has been transmitted through the objective lens 100 is transmitted through the light transmitting layer 5 of each of the optical discs 200 to be converged on the information layer 50.
Referring to FIGS. 8 and 9, a configuration example of a conventional optical pickup and a configuration of the objective lens 100 included in the optical pickup are described below.
First, referring to FIG. 8, the configuration example of the known optical pickup is described. For simplicity, FIG. 8 illustrates only a configuration on a forward path side (side from the light source to a disc surface), and a configuration on a return path side (side from the optical disc to a photodetector) is omitted. In the example illustrated in FIG. 8, first, a case is assumed where a BD is loaded in the optical disc device. In this case, blue light (having a wavelength of 0.405 μm) is emitted from a light source 6a such as blue-light emitting semiconductor laser. The blue light beam emitted from the light source 6a is reflected on a dichroic mirror prism 7, and travels through a collimating lens 8 so as to be converted into plane waves 4a. The dichroic mirror prism 7 is configured to reflect light having the blue wavelength, and transmit light having red and infrared wavelengths. The optical system of this example causes the plane waves 4a to enter the objective lens 100, and hence is an “infinite system”. The blue light beam travels through the objective lens 100, is transmitted through the light transmitting layer 5 having a thickness of 0.1 mm, and is converged on the information layer 50. The BD includes, as illustrated in FIG. 7A, an optical disc base 200 having a thickness of 1.1 mm and the light transmitting layer 5 having a thickness of 0.1 mm and the information layer 50 is positioned between the disc base and the light transmitting layer 5. The light transmitting layer in the BD is constituted of a protective layer having a thickness of 0.1 mm. Note that, a DVD has a structure in which a pair of disc bases each having a thickness of 0.6 mm are bonded together, and the information layer 50 is positioned between the pair of disc bases. Therefore, the light transmitting layer 5 in the DVD corresponds to one disc base having the thickness of 0.6 mm, and hence the light transmitting layer thickness is 0.6 mm. On the other hand, a CD includes a disc base having a thickness of 1.2 mm, and the information layer 50 is positioned on a rear surface side of the disc base. Therefore, the light transmitting layer 5 in the CD corresponds to the disc base having the thickness of about 1.2 mm, and hence the light transmitting layer thickness is 1.2 mm.
When a DVD or a CD is loaded in the optical disc device, the light beam having a red wavelength or the light beam having an infrared wavelength is emitted from a light source 6b. The light source 6b includes a red semiconductor laser and an infrared semiconductor laser arranged in one package. The red or infrared light (having a wavelength of 0.660 μm or 0.785 μm) emitted from the semiconductor laser light source 6b for independently emitting two wavelengths of red and infrared is transmitted through the dichroic mirror prism 7, and travels through the collimating lens 8 to be converted into the plane waves 4a. Thereafter, the red or infrared light beam travels through the objective lens 100 and is transmitted though the light transmitting layer 5 having a thickness of 0.6 or 1.2 mm to be converged on the information layer 50. The optical system in this example also causes the plane waves 4a to enter the objective lens 100, and hence is an “infinite system”.
The objective lens 100 is formed of a transparent medium (such as glass or plastic), and has a center axis corresponding to an optical axis L. The objective lens 100 includes a surface on which a grating 1a having a sawteeth-like cross section is formed. Phase step portions of the grating 1a are arranged concentrically around the optical axis L. The grating 1a is set so as to diffract the blue wavelength in the 3rd order and diffract the red and infrared wavelengths in the 2nd order.
FIG. 9A illustrates a part of the cross-sectional shape of the grating in the conventional example. The grating 1a having a depth d1 diffracts incident light 4a to obtain diffracted light 4b. FIG. 9B shows a relationship of a diffraction efficiency with respect to a wavelength in a case where the transparent medium forming a lens base 1 of the objective lens 100 is Zeonex 350 (having a refractive index nd=1.50620 and an Abbe number ud=56.3877) and the depth d1 of the grating 1a is 2.58 μm. When the wavelength is 0.405 μm, the efficiency of the 3rd-order diffracted light is 69.5%, and the efficiencies of the 2nd-order diffracted light are 99.7% and 65.2% for the wavelengths of 0.660 and 0.785 μm, respectively.
Next, the objective lens 100 in this conventional example is considered in terms of aberrations. A case is assumed where an optical disc has a light transmitting layer thickness of x and an objective lens is designed to cancel the spherical aberration (base aberration) occurring from the light transmitting layer thickness. By adding a grating to the lens, the base aberration and the spherical aberration of the light transmitting layer thickness are both absorbed. In this case, it can be considered that the spherical aberration corresponding to the light transmitting layer thickness (0.1−x) is added to the light having the blue wavelength λ1 due to the diffraction, the spherical aberration corresponding to the light transmitting layer thickness (0.6−x) is added to the light having the red wavelength λ2, and the spherical aberration corresponding to the light transmitting layer thickness (1.2−x) is added to the light having the infrared wavelength λ3. In practice, there are dispersion effects of the lens material and the disc base, and as the wavelength becomes shorter, the refractive index of the disc or the lens becomes larger and the difference in spherical aberrations becomes larger at the same time. When the increase in spherical aberrations is converted into a difference in light transmitting layer thickness and aberration change amounts for the blue wavelength λ1, the red wavelength λ2, and the infrared wavelength λ3 are denoted by t1, t2, t3 (provided that the infrared wavelength is used as a reference of dispersion and is set to t3=0), the aberration influences on the positive side of the difference in light transmitting layer thickness more as the wavelength becomes shorter, to thereby satisfy the condition t1>t2>t3=0. Therefore, the diffraction adds the spherical aberration corresponding to the light transmitting layer thickness (0.1−x+t1) to the light having the blue wavelength λ1, the spherical aberration corresponding to the light transmitting layer thickness (0.6−x+t2) to the light having the red wavelength λ2, and the spherical aberration corresponding to the light transmitting layer thickness (1.2−x+t3) to the light having the infrared wavelength λ3.
On the other hand, the phase change (aberration) that occurs due to the diffraction is proportional to the diffraction order multiplied by the wavelength, and hence, when the diffraction order at the grating is the pth-order for the blue wavelength λ1, the qth-order for the red wavelength λ2, and the rth-order for the infrared wavelength λ3, the following equation is satisfied:(0.1−x+t1):(0.6−x+t2):(1.2−x+3)=pλ1:qλ2:rλ3.  (Equation 1)
When it is assumed that q=r, λ1=0.405 μm, λ2=0.660 μm, and λ3=0.785 μm and it is approximated that t2=t3=0 because the dispersion is small for red or longer wavelengths, x=−2.57 mm from (Equation 1) and the following equation is established:p/q=660(0.1−x+t1)/405(0.6−x)=1.37+0.51*t1  (Equation 2)where t1 is a positive value and can be adjusted from below 0.1 to about 0.2 by a variation value of the lens material. For example, p/q≈1.4 when a material having a low dispersion is used, but may be increased by using a material having a high dispersion to p/q≈1.5. A combination that is closest to the condition of (Equation 2) and lowest in order is p=3 and q=r=2. In other words, the relationship of p=3 and q=r=2 is the condition that can minimize the aberration occurring from the difference in light transmitting layer thickness.
As described above, in the optical disc device of the conventional example, even with the configuration in which light 4a parallel to the objective lens 100 is caused to enter, the aberrations occurring from the three different light transmitting layer thicknesses of the discs may be minimized, and a certain light utilization efficiency (diffraction efficiency) may be maintained for the three different wavelengths. Further, the light 4a incident on the objective lens 100 may be converted into the plane waves, and hence the positional adjustment between the objective lens 100 and the incident light 4a is facilitated and the aberration occurring from the tracking shift of the objective lens 100 may be suppressed. Therefore, a smaller and cheaper optical system may be provided.